The present invention is directed to a cement admixture composition capable of causing the combined effects of inhibiting drying shrinkage of cement compositions while permitting substantial air entrainment to be imparted thereto. The present invention further provides an improved cement composition structural product and a method of forming same.
Specifically, the present invention is directed to a cement admixture composed of a synergistic combination of certain monoalkyl ether alkylene oxide adducts and alkylene glycols, as fully described hereinbelow.
Hydraulic cement compositions, such as mortar (cement, small aggregate, e.g. sand, and water), or concrete (cement, small aggregate, large aggregate, e.g. crushed stone and water), have certain properties which substantially affect their durability. These properties include shrinkage which normally occurs during drying of the cement composition and the amount of air entrained in the resultant cast cement composition's structure.
Conventional hydraulic cement compositions display a decrease in volume with setting and drying of the cast composition. Although the magnitude of the volume decrease is normally small, it is of extreme importance. This shrinkage results in cracks and other defects which lower the serviceability and durability of the resultant structure. The cracks provide a path for air to penetrate into the structure, promoting carbonation of the cement and corrosion of the metal reinforcing bars contained therein. Further, the cracks provide a means for water to seep into and through the structure. Such water entry further deteriorates the structure through freeze-thaw cycling pressures exerted on the cement structure over its life. It is highly desired to provide a cement which exhibits high strength and is not subject to deterioration effects due to shrinkage and freeze-thaw cycling.
Various attempts have been made to avoid the cracking phenomenon caused by drying shrinkage. These include providing joints in the cement structure to concentrate the site of crack formation at the joint and, thereby, minimize such formation at other portions of the structure. Such joints are expensive to install; are not applicable to certain structures such as vertical walls, pillars and the like; and merely concentrate the area of cracking but do not alleviate it.
Other attempts include varying the composition of the cement, varying the methods of manufacture of concrete mix and varying the ballast material used in forming the resultant concrete structure. None of these attempts have resulted in a satisfactory solution. For example, cements have been formulated with expansive admixtures in attempts to counter the shrinkage of the concrete. However, it is difficult to determine the proper amount of expansive admixture required to counter the drying shrinkage which develops. The use of such materials thereby give rise to unpredictable results.
With respect to overcoming the drying shrinkage of cement compositions, such as concrete compositions, the literature teaches that various oxyalkylene adducts are suitable for this purpose. For example, U.S. Pat. Nos. 3,663,251 and 4,547,223 suggest the use of compounds of the general formula RO(AO).sub.n H in which R may be a C.sub.1-7 alkyl or C.sub.5-6 cycloalkyl radical, A may be C.sub.2-3 alkylene radicals and n is 1-10, as shrinkage reducing additives for cement. Similarly, U.S. Pat. No. 5,147,820 suggests terminally alkyletherified or alkylesterified oxyalkylene polymers as useful for shrinkage reduction. Still further, Japanese Patent Application 58-60293 provides the suggestion that shrinkage reduction of cement can be accomplished by the addition thereto of compounds which are aliphatic, alicyclic or aromatic group terminated oxyethylene and/or oxypropylene repeating chain compounds.
The freeze-thaw pressures encountered by conventional hydraulic cement structures on a micro-scale (including crack phenomenon) is due to seepage of water into the porous cement structure where it resides to exert deteriorating pressure under freeze-thaw conditions. In order to prevent the loss of durability due to this phenomenon, it is common practice to incorporate small amounts of agents capable of causing the entrainment of fine air voids in the hardened hydraulic cement composition structure (Air entraining agents or AE agents). These air voids (normally 3-10, preferably 4-8 volume percent) provide empty space for expansive ice crystals to grow into relieving the pressure of water expansion under freeze-thaw conditions.
While oxyalkylene compounds provide a degree of shrinkage inhibition to cement structures, they have been known to cause a deactivation of conventional air entraining agents and, therefore, cause such treated cement structures to have an undesired low degree of air entrainment and/or require a very high dosage of AE agent (which has many disadvantages from a practical and economic standpoint). It is known that air entrainment is desired to aid in permitting the cast cement structure to withstand the compression/expansion forces encountered. The alkylene oxide adducts described above do not permit the structure to have sufficient air entrainment, as required, to provide a structure capable of withstanding compressive/expansive forces and, thereby, extend the structure's useful life. For example, U.S. Pat. No. 3,663,251 shows, by comparative examples, that the inclusion of a polypropylene glycol causes a reduction of the air entrainment provided by the cement being treated with an agent composed of sulfite waste liquor. Further, Canadian Patent 967,321 suggests that polyoxyalkylene glycols as well as their esters, ethers and mixture cause a reduction in foaming in cementitious compositions.
It is highly desired to provide a cement admixture which is capable of inhibiting drying shrinkage of structural cement compositions without causing a reduction in the ability of conventional air entraining agents to impart sufficient air thereto.